This invention relates generally to a rapid thermal heating apparatus and method for rapidly heating a substrate, and more particularly to a radiant energy heating source having improved controllability of the spatial heating of a substrate by reflecting energy from a radiant energy source.
Single-wafer processing of semiconductors is a powerful and versatile technique for the fabrication of very-large-scale integrated (VLSI) and ultra-large-scale integrated (ULSI) electronic devices. It combines low thermal mass photon-assisted rapid wafer heating with reactive ambient semiconductor processing. Both the wafer temperature and the process environment can be quickly changed (because of short transient times) and, as a result, each fabrication step and its sub-processes can be independently optimized in order to improve the overall performance of the fabricated devices.
Rapid thermal processing (RTP) of semiconductor wafers provides a capability for better wafer-to-wafer process repeatability in a single-wafer lamp-heated thermal processing reactor. Numerous silicon fabrication technologies use RTP techniques, including rapid thermal annealing (RTA), rapid thermal cleaning (RTC), rapid thermal chemical vapor deposition (RTCVD), rapid thermal oxidation (RTO), and rapid thermal nitridation (RTN). For example, RTCVD processes to form dielectrics (e.g., oxides and nitrides), semiconductor materials (e.g., amorphous silicon and polysilicon), as well as conductors (e.g., aluminum, copper, tungsten, and titanium nitride) can be performed using advanced RTP techniques for VLSI and ULSI device fabrication.
In the semiconductor industry, it is desirable to obtain temperature uniformity over the surface of each substrate during temperature cycling of substrates. Surface temperature uniformity provides uniform process variables (e.g., layer thickness, resistivity and etch depth) for various temperature-activated steps such as film deposition, oxide growth and annealing. In addition, temperature uniformity is necessary to prevent thermal stress-induced damage such as warpage, defect generation and slip.
In the particular application of CMOS gate dielectric formation by RTO or RTN, thickness, growth temperature, and uniformity of the gate dielectrics are critical parameters that influence the overall device performance and fabrication yield. Currently, CMOS devices are being made with dielectric layers that are only 60-80 Angstroms thick and for which thickness uniformity must be held within +/- 2 Angstroms. This level of uniformity requires that temperature variations across the substrate during high temperature processing cannot exceed a few degrees Centigrade (.degree. C.).
The wafer itself often cannot tolerate even small temperature differentials during high temperature processing. If the temperature differential is allowed to rise above about 1-2.degree. C., at 1200.degree. C., then the resulting stress is likely to cause slip in the silicon crystal. The resulting slip planes will destroy any devices through which they pass.
Equipment manufacturers have spent significant design resources to insure uniform wafer heating in RTP systems. For example, U.S. Pat. No. 5,155,336 ('336 patent), assigned to the assignee of the subject application, discloses an RTP chamber that utilizes a plurality of radiant energy sources arranged to supply heat to a substrate. The radiant energy sources are positioned so that irradiated areas of the substrate corresponding to adjacent radiant energy sources overlap. This overlap is caused by overlapping distributions of the radiant energy density - - - also referred to as energy flux distributions and measured in units of energy/area - - - of each radiant energy source. The overlap helps to ensure that the entire surface of the substrate is irradiated uniformly.
Relative radiation intensity across a substrate is dependent upon the overlapping distributions from adjacent radiant energy sources. Additional radiation, however, is contributed to portions of the substrate's surface from non-adjacent radiant energy sources through multiple reflections of radiant energy off the surface of the substrate and the surfaces of the RTP chamber. The more reflective the surface of the substrate, the more energy is reflected off the substrate surface to be transmitted about the RTP chamber, to contact the substrate's surface at locations remote from the radiant energy's source.
As a result of the overlap of flux distributions and the multiple reflections of radiant energy, different radiant energy sources contribute in varying degrees to the radiation density at any one point on the substrate surface. This fact complicates the process of controlling the temperature profile across the substrate's surface through manipulating the energy supplied to the various radiant energy sources.
It has also been recognized that a trade-off occurs between the controllability of the temperature uniformity across the substrate surface and the efficiency of the radiant energy sources. For example, in an RTP chamber, as the energy flux distribution from a radiant energy source is restricted by, for example, narrowing the outlet of the source's light pipe to achieve better controllability of the substrate's surface temperature, the efficiency of the radiant energy source decreases because less energy is directed toward the substrate. Conversely, if the efficiency of the radiant energy source is increased by increasing the size of the outlet, and necessarily the distribution of radiant energy released from the radiant energy source, the controllability decreases because of the increased multiple reflections and flux distribution overlaps, described above. Accordingly, it would be desirable to improve the efficiency of the radiant energy sources while not decreasing the controllability of the distribution of the radiant energy directed toward the substrate.
It is a general object of this invention to provide an improved radiant energy heating source for rapid thermal processing of substrates such as semiconductor wafers.
It is further object of this invention to provide a radiant energy heating source which allows improved spatial control of radiant energy applied to a substrate while realizing improved radiant energy source efficiency.
It is another object of this invention to provide a radiant energy heating source which includes a plurality of reflectors having a tapered portion at the light emitting end which direct energy onto pre-determined overlapping areas of a substrate.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out in the claims.